Add support for custom label font and color, close button and auto hide.

[cherifya]

Hi,

Great class. Such a component is sorely missing on OSX. I added a few bits I needed :

• custom label font and color
• an optional close button
• support for auto hiding the overlay after a given delay

I also cleaned up the code and xcode project to separate the core files from the sample app. To make it easier for inclusion in Cocoapoads ;)

Cheers, Cherif

N.B: Here's an updated capture.